If you don't understand "description" then you need to take some English classes.

Rafaelluik gave you the right solution, or at least the only one that I know of. That works to open background tabs next to the active tab for every method that I tried (mouse gesture, middle-click on link, right-click menu) except for double-clicking on the tab bar, so you need to describe how you opened a new tab in the background that did not open next to the active tab when the right setting was used. Otherwise, no one will know what you mean by “doesn't work.”